Field of the Invention
The present invention relates to a display device, and more particularly to a display device capable of appropriately detecting a touch operation by a user.
Description of the Background Art
General image forming apparatuses such as MFPs (Multi Function Peripherals) are provided with operation panels. An operation panel includes a touch panel which displays software keys and a variety of information for accepting operations through the user's operating finger, and hardware keys provided around the periphery of the display. An image forming apparatus accepts a touch from a user on a software key appearing on the touch panel to execute a process allocated to the key. The touch panel allows the user to perform an operation by directly touching a display screen appearing on the touch panel, thereby enabling an intuitive operation and achieving high operability.
The operation panel of a multi-function image forming apparatus, typically, such as an MFP, has a display configuration in which a preview image and a number of software keys are fitted in a single screen on the operation panel. The operation panel therefore tends to display software keys and characters on the keys in a reduced size. Most of operation panels therefore are provided with a screen zoom function for enlarging a screen.
The touch panel technology has recently evolved with proliferation of smart phones, tablet terminals, and other devices, and touch panels are capable of multi-touch. Multi-touch refers to an operation of simultaneously touching a plurality of points on a touch panel. Examples of multi-touch include pinch gestures. Examples of the pinch gestures include pinch-out in which two closed fingers are opened, and pinch-in in which opened fingers are closed.
Document 1 discloses an information processing apparatus in which a screen in an enlarged or reduced size appears when a pinch gesture is accepted. The information processing apparatus includes a touch position detection unit and a display unit. When touches at any two positions are detected simultaneously, the touch position detection unit additionally detects a change in the distance between touch-detected positions. The display unit enlarges the size of information appearing on the display unit in accordance with the amount of increase in the distance between the touch-detected positions and reduces the size of information appearing on the display unit in accordance with the amount of decrease in the distance between the touch-detected positions. This information processing apparatus allows the user to enlarge or reduce a screen on the operation panel through such an intuitive operation as a pinch gesture.
To perform a pinch gesture on a touch panel, a user starts a pinch gesture by simultaneously touching any two points on the touch panel with two fingers. It is, however, difficult to simultaneously touch the touch panel with two fingers. For example, the difference in length between two fingers, the angle or height of the touch panel, the user's dominant hand or manner of motion of fingers, or other factors may cause a subtle time lag in touch timing although the user intended simultaneous touch with two fingers. The information processing apparatus according to Document 1 below may erroneously determine that a touch by one of the user's two fingers touching the touch panel that comes into contact first is single touch although the user intended to perform a pinch gesture.
FIG. 31 is a diagram illustrating a touch determination method in a conventional information processing apparatus. It is noted that in FIG. 4, FIG. 6, FIG. 8, FIG. 12, FIG. 14, FIG. 15, FIG. 23, and FIG. 31 the touch positions on the touch panel are hatched.
Referring to FIG. 31, assume that the user of the information processing apparatus makes a first touch at a position P101 on the touch panel and makes a second touch at a position P102 on the touch panel slightly later than the first touch thereby to perform multi-touch on the touch panel. In this case, the information processing apparatus may erroneously determine that the touch at position P101 is single touch and perform a process that would be performed when a key KY100 at position P101 was pressed, and may ignore the second touch.
Document 2 below discloses a technique that can prevent an erroneous determination that a pinch gesture is single touch. This touch operation device counts the number of touch points including the initial touch point that are sensed by the touch panel within a predetermined measurement time from the start of the initial touch, and determines that the counted number is the collective number of touch points.
[Document 1] Japanese Laid-Open Patent Publication No. 2007-207275 (U.S. Pat. No. 4,412,348)
[Document 2] Japanese Laid-Open Patent Publication No. 2011-70250
With the touch operation device in Document 2, it is difficult to set a length of the measurement time. Specifically, the number of touch points cannot be finalized unless a predetermined measurement time has passed since the start of touch at the first touch point, so that the responsiveness is reduced when the measurement time is long. The accuracy of detection of touch points is reduced when the measurement time is short.
These problems are not unique to operation panels and may generally occur in display devices including portable terminals. In a portable terminal, when a pinch gesture is accepted on a browser screen, the browser screen is enlarged or reduced, and when selection of a link is accepted through single touch (single tap), the screen makes a transition to the link. The information terminal thus may erroneously determine that a first touch of multi-touch is single touch. Presumably, some information terminals that detect a touch based on off-edge (a change from a state in which the display is touched to a state in which the display is not touched) do not have such a problem.